The present invention relates generally to electrical circuits and components, and more particularly to a method and apparatus for regulating voltage in high power applications.
Many electronic components require a constant, stable direct current (DC) voltage source to operate properly. A voltage regulator is a type of electrical component that provides stable output voltage and variable output current. While a voltage regulator may be powered by a variable or unstable voltage source, a constant, stable voltage is available at the output of the voltage regulator.
A linear regulator is one type of voltage regulator that includes a linear control element and an electrical feedback element. The linear control element, often a transistor, is coupled in series with the unregulated input voltage. The feedback element is used to maintain a constant output voltage by comparing the output voltage to a stable, known voltage reference. The voltage drop across the linear control element is varied so the output voltage remains equal to the reference voltage, even while the input voltage varies. The output voltage is always lower than the unregulated input voltage as some power is dissipated in the control element. A shunt regulator is a type of linear regulator in which the linear control element is tied from output to ground rather than in series with the load. Linear regulators are advantageous because they respond very quickly to fluctuations in load current and input voltage. However, linear regulators often must dissipate a great deal of power, equal to the output current multiplied by the difference between the input and output voltages. Large power dissipation requires adequate cooling for proper operation of the regulator and surrounding components, necessitating a relatively large device or an additional cooling element.
Another type of voltage regulator is a switching regulator. A switching regulator includes a transistor operated as a saturated switch. The transistor applies the full unregulated input voltage across an inductor for short intervals. As the current builds up, the energy stored in the inductor is transferred to a filter capacitor at the output of the device. The output voltage is compared to a voltage reference and feedback is used to vary the pulse width and/or frequency of the periodic application of power by the transistor. Since switching regulators are either off or saturated, they dissipate very little power, which permits them to operate very efficiently and, therefore, to be relatively small and light. However, switching regulators respond relatively slowly to abrupt changes in current load or input voltage. In a switching regulator, many pulses of current through the inductor are required to compensate for an abrupt change in a current load or input voltage.
In a first aspect, the invention features a voltage regulator having a regulator output operating at a predetermined normal output voltage. The voltage regulator includes a switching regulator that has a first input and a first regulated output providing a normal first output voltage level, where the normal first output voltage level is approximately equal to the predetermined normal output voltage. The first regulated output is coupled to the regulator output. The voltage regulator includes a linear regulator that has a second input and a second regulated output providing a second normal output voltage level. The second normal output voltage level is less than the first normal output voltage level. The second regulated output is coupled to the first regulated output such that the voltage at the regulator output is maintained at approximately the predetermined normal output voltage by operation of the switching regulator until the predetermined normal output voltage falls below the second normal output voltage level at which time the regulator output is maintained at approximately the predetermined normal output voltage by operation of either the linear regulator or both the linear regulator and the switching regulator.
Implementations of the invention may include one or more of the following features. The second normal output voltage level may be approximately 1.5% less than the first normal output voltage level. The normal output voltage of the voltage regulator may be approximately 1.5 V.